Nuclear envelope breakdown (NEBD) is an essential process of both the mitotic and meiotic cell cycle. The mechanisms regulating NEBD will be investigated by imaging the spatial and temporal dynamics of green fluorescent protein chimeras of cyclin, nuclear pore proteins, and lamin in living starfish oocytes. Starfish oocytes will be used because they are optically clear, and have a large nucleus that undergoes NEBD at a precisely defined time (20-30 min) after application of a hormone. Confocal microscopy and photobleaching techniques will be used to address three specific aims: 1) to investigate mechanisms by which cyclin B/cdc2 enters the nucleus, 2) to investigate the relationship between nuclear pore disassembly and the permeability increase that precedes NEBD, and 3) to investigate the mechanisms of disruption of the nuclear envelope membrane. These studies should provide information about the fundamental question of the regulation of the cell cycle.